[time-nuts] Characterising frequency standards

[Steve Rooke]

2009/4/13 Magnus Danielson <magnus at rubidium.dyndns.org>:
>>> Dead time is when the counter looses track of time in between two
>>> consecutive measurements. A zero dead-time counter uses the stop of one
>>> measure as the start of the next measure.
>>
>> This becomes very important when the data to be measured has a degree
>> of randomness and it is therefore important to capture all the data
>> without any dead time. In the case of measurements of phase error in
>> an oscillator, it should be possible to miss some data points provided
>> that the frequency of capture is still known (assuming that accuracy
>> of drift measurements is required).
>
> Depending on the dominant noise type, the ADEV measure will be biased.

If the noise has a component related to the measurement frequency,
agreed, but I have already commented on that before.

>> Indeed, there would be a loss of statistical data but this could be
>> made up by sampling over a period of twice the time. This system is
>> blind to noise at 1/2 f but ways and means could be taken to account
>> for that, IE. taking two data sets with a single cycle space between
>> them or taking another small data set with 2 cycles skipped between
>> each measurement.
>
> Actually, you can take any number of 2 cycle measures and be unable to
> detect the 1/2 f oscillation without detecting it. In order to be able
> to detect it you will need to take 2 measures and be able to make an odd
> number of cycles trigger difference between them to have a chance.

Agreed.

> The trouble is that the modulation is at the Nyquist frequency of the 1
> cycle data, so it will fold down to DC on sampling it at half-rate.
> Canceling it from other DC offset errors could be challenging.

Comparing the frequency calculated from the data would show a 2Hz
offset with the fundamental frequency of the source.

> Sampling it at 1/3 rate would discover it thought.

Agreed.

>> I'm looking at what can be acheieved by a budget strapped amateur who
>> would have trouble purchasing a later counter capable of measuring
>> with zero dead time.
>
> Beleive me, that's where I am too. Patience and saving money for things
> I really want and allowing accumulation over time has allowed me some
> pretty fancy tools in my private lab. Infact I have to lend some of my
> gear to commercial labs as I outperform them...

Well, that's a goal for me but I'm looking at what is achievable in
the short term instead of sitting on my hands.

> I recalled wrong. You should look for Barnes "Tables of Bias Functions,
> B1 and B2, for Variance Based on Finite Samples of Processes with Power
> Law Spectral Densities", NBS Technical Note 375, Janurary 1969 as well
> as Barnes and Allan "Variance Based on Data with Dead Time Between the
> Mesurements" NIST Technical Note 1318, 1990.
>
> A ahort into to the subject is found in NIST Special Publication 1065 by
> W.J. Riley as found on http://www.wriley.com along other excelent
> material. The good thing about that material is that he gives good
> references, as one should.

Thanks for the pointer.

>> I could look at doing that perhaps.
>
> You should have two counters of equivalent performance, preferably same
> model. It's a rather expensive approach IMHO.

It may still be cheaper than the purchase of a counter capable of
continuous collection, especially if you already have a counter that
is capable at 1/2 f.

> Have a look at the possibility of picking up a HP 5371A or 5372A. You
> can usually snag one for about 600 USD or 1000 USD respectively on Ebay.

I'd have to be a really good boy for Santa to bring me something of
that ilk. Perhaps the lotto will come up one day :-)

73,
Steve

> Cheers,
> Magnus
>
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-- 
Steve Rooke - ZL3TUV & G8KVD & JAKDTTNW
Omnium finis imminet